Process for producing a high magnetic flux density grain-oriented electrical steel sheet

ABSTRACT

Process for producing a high magnetic flux density grainoriented electrical steel sheet comprising; subjecting silicon steel ingot containing 0.005 to 0.1% of carbon, 2.5 to 4.0% of silicon, 0.01 to 0.10% of selenium and one of 0.01 to 0.15% of arsenic, 0.02 to 0.3% of bismuth, 0.02 to 0.3% of lead, and 0.02 to 0.2% of antimony with a sulfur content low enough to be nonharmful to desired magnetic properties to cold rolling in which the reduction of the final cold rolling step is not less than 70%, and then to decarburization and final annealing.

United States Patent lchiyama et al.

[451 Sept. 30, 1975 PROCESS FOR PRODUCING A HIGH MAGNETIC FLUX DENSITYGRAIN-ORIENTED ELECTRICAL STEEL SHEET Inventors: Tadashi lchiyama,Sagamihara;

Takashi Sato; Tsuyoshi Kikuchi, both of Kawasaki, all of Japan Assignee:Nippon Steel Corporation, Japan Filed: Mar. 14, 1974 Appl. No.: 451,231

Foreign Application Priority Data Mar. 20, 1973 Japan 48-31452 U.S. C172/365; 148/111 Int. C1. B2113 3/02 Field of Search 148/110, 111;72/700, 366,

References Cited UNITED STATES PATENTS 10/1967 Dctcrt 148/111 3,802,9364/1974 Goto et a1 148/111 Prinulry ExaminerLowell A. Larson Attorney,Agent, or Firm-Toren, McGeady and Stanger nium and one of 0.01 to 0.15%of arsenic, 0.02 to 0.3% of bismuth, 0.02 to 0.3% of lead, and 0.02 to0.2% of antimony with a sulfurcontent low enough to be non-harmful todesired magnetic properties to cold rolling in which the reduction ofthe final cold rolling step is not less than 70%, and then todecarburization and final annealing.

2 Claims, 2 Drawing Figures PROCESS FOR PRODUCING A HIGH MAGNETIC FLUXDENSITY GRAIN-ORIENTED ELECTRICAL STEEL SHEET The present inventionrelates to commercial production of grain-oriented electrical steelsheets having a very high degree of integration of the so-called Gossstructure having the magnetizable axis 100 along the rolling directionof the sheet and the (1 l) plane in the surface of the steel sheet.

Grain-oriented electrical steel sheet has been used mainly for ironcores for electrical appliances such as, transformers and other, whereingood excitation characteristics and good watt loss properties arerequired. The excitation characteristic is expressed by the value of themagnetic flux density in a magnetic field of 800A/m, and the watt lossproperties are expressed by the electricity loss at a magnetic fluxdensity of 17 K Gauss.

Methods for producing the high magnetic flux density, grain-orientedelectrical steel sheet having good excitation and watt losscharacteristics as well as good magnetostriction characteristics weredisclosed by the present inventors in Japanese Patent Applications Sho45-92277 and Sho 46-82442. According to the proposed methods, siliconsteel ingots are produced by adding one or more of arsenic, bismuth,lead and antimony together with small amount of selenium using theconventional steel making and ingot casting methods. The hot rolledsteel sheet obtained from the ingot is subjected to a cold rollingprocess including a final cold rolling step of more than 80% reductionso as to produce a high magnetic flux density grain-oriented electricalsteel sheet having B characteristics of more than 1.85 wb/m The presentinvention relates to improvements of the above methods disclosed in theJapanese Patent Applications Sho 45-92277 and Sho 46-82442. Namely, thepresent invention defines additional conditions for the constituentelements added in the starting materials, and by these additionalconditions, the present inventors have succeeded in improving theexcitation and watt loss characteristics and in reducing fluctuation inthe excitation characteristics of the final products.

The present invention will be described referring to the attacheddrawings.

FIG. I shows the relation between the sulfur contents and the excitationcharacteristics in a 3% silicon steel containing one of four elementsfrom the group of arsenic, bismuth, lead and antimony, in addition toselenium in case of the reduction of not less than 70% in the final coldrolling step.

FIG. 2 shows similarly the relation between the sulfur contents and theB values when the final cold rolling step is performed at a reductionrate between 50 and 60%.

In FIG. 1, the values of magnetic flux density B at a 'magnetic field of800A/m are plotted against the sulfur than 0.01%, the B value becomesremarkably high and the fluctuation is also improved. When a specialtreatment is applied, a normal sulfur content in the steel ingot isusually between 0.015 and 0.025%. Therefore, in order to maintain thesulfur content to not more than 0.01% and obtain this remarkableimprovement of the magnetic properties, it is necessary to lower thesulfur content by using special methods, such as, the rotary kilnmethod, the DM converter method and the shaking ladle method.

I In FIG. 2, the relation between the sulfur contents and the value ofthe magnetic flux density B when the reduction of the final cold rollingstep is between 50 and 60% which is a normal reduction in the productionof grain-oriented electrical steel sheets is shown. It is understoodfrom FIG. 2 that the lowering of the sulfur content does not improve themagnetic flux density value, but rather increases the fluctuation of thevalue. This means that the new additionalcondition of the sulfur contentamong the constituent elements is effective when the reduction of thefinal cold rolling step is higher than the normal reduction of 50 to60%.

The lowering of the sulfur content to not more than 0.01% improves the Bcharacteristics of the magnetic flux density not only in the case of thecombination addition of one of arsenic, bismuth, lead, and antimony withselenium, but also in the case of the combination addition of two ormore of arsenic, bismuth, lead and antimony with selenium as disclosedin Japanese Patent Application Sho 46-82442. Based on the above facts,

the chemical composition of the starting material used in the presentinvention has been defined as follows:

Carbon 0.005 to 0.1%,

Silicon 2.5 to 4.0%,

Sulfur not more than 0.01%, and Selenium 0.01 to 0.1%, with one elementselected from the group of Arsenic 0.01% to 0.15%, Bithmus 0.02 to 0.3%,Lead 0.02 to 0.3%, and Antimony 0.02 to 0.2%.

When two or more of the last four elements are added, the total amountshould be between 0.02 and 0.5%, with the balance being iron and tracesof impurities which are harmless to the magnetic properties.

The reasons for the above limitations of the various elements in thesteel are as follows. When the silicon content is less than 2.5%, the a'y transformation takes place irrespective of the carbon contentwhatsoever, so that growth of the secondary recrystallization grainstake place during the final annealing which is normally done at atemperature not lower than 1000C. On the other hand, silicon contents ofmore than 4.0% tend to cause crackings due to embrittlement during thecold rolling. Thus the silicon content is defined to 2.5 to 4.0% in thepresent invention.

Carbon is necessary for promoting the presence of a precipitateddispersion containing arsenic, bismuth, lead or antimony, and when thetotal amount of the elements constituting the precipitated dispersion issmall, the carbon content may be in the higher side in the range andwhen the total amount is large, it may be in the lower side. 7

Each of arsenic, bismuth, lead and antimony, when present together withselenium, forms a fine dispersion and is necessary for the secondaryrecrystallization, after the final annealing, to be attained with astrong reduction of at least 70% in the final cold rolling. Sufficientdispersion can not be obtained if the contents of these elements arelower than the lower limit of these ranges. Although their contents maybe safely beyond the upper limits, such larger contents cause increasedproduction cost.

Although no theoretical explanation can be given for the fact that theexcitation and watt loss characteristics of the final products can beremarkably improved when the sulfur content in the starting siliconsteel ingot is maintained at not more than 0.01%, the present inventorshave the following comments.

The excitation and watt loss characteristics of a grain-orientedelectrical steel sheet depend mainly on the integration degree of theGoss structure formed by the secondary recrystallization which takesplace during the final annealing.

Thus, a higher degree of the Goss structure assures better excitationand watt loss characteristics. In order to obtain a high degreeintegration of the Goss structure, it is necessary that the reduction inthe final cold rolling step is higher than the normal reduction of 50 to60% and that the dispersion which shows special thermal behavior nearthe recrystallization temperature is utilized. For obtaining thisdispersion, the combined addition of arsenic, bismuth, lead and antimonywith selenium is effective as disclosed in Japanese Patent ApplicationsSho 45-92277 and Sho 46-82442. But in this case, sulfur, such as, MnS,which forms dispersion showing a different thermal behavior is harmful.Thus, in order to permit extraordinary growth of only the primaryrecrystallization having the ideal Goss orientation produced by the highcold rolling reduction, it is considered to be necessary that the stateof the dispersion is changed sharpy near the starting temperature of thesecondary recrystallization. For this purpose, it is not desirable touse a plurality of dispersions having different thermal behaviors.

According to the present invention, silicon steel ingots having achemical composition falling within the above defined range are producedby a conventional method and hot rolledinto hot rolled steel sheets of1.5 to 5.0 mm thickness. In this case, the magnetic properties of thefinal products are considerably improved when the steel material isretained for 60 to 360 seconds within a temperature range of l200 to850C in the course of the continuous hot rolling step. The hot rolledsteel sheet, after acid pickling, is subjected one time or more than twotimes to the cold rolling step in such a-manner that the reduction ofthe final cold rolling is not less than 70%. In this case, when thesheet is subjected more than two times to the cold rolling step, thesheet is subjected to an intermediate annealing for l to 30 minutes at atemperature between 750 and 1200C in a neutral or reducing gas betweenthe cold rollings. When the final sheet thickness is to be obtained by asingle cold rolling step, it is desirable to subject the hot rolledsteel sheet to a preliminary annealing for l to 30 minutes at atemperature between 750 and l200C in a neutral or reducing gas prior tothe cold rolling.

The reason for defining the reduction of the final cold rolling as notless than 70% is to obtain a high magnetic flux density B of more than1.85 wb/m However it has been found that a reduction rate of not lessthan 60% is enough due to the considerably improved level of themagnetic properties by the improvement of the hot rolling conditions asdescribed above.

The steel sheet which has been reduced into the final thickness by thecold rolling is subjected to a short-time annealing by a conventionalmethod for the purpose of decarburization and primary recrystallization.For example, the sheet is annealed in a gas mixture of wet hydrogen andnitrogen at a temperature between 800 and 850C for a few minutes.Finally the sheet is annealed in a reducing gas at a temperature of morethan 1000C, preferably more than ll00C, for more five hours. During theannealing, elements harmful to the watt loss characteristics, such as,carbon, selenium, and the remaining sulfur are removed and the secondaryrecrystallization proceeds so that a grain-oriented electrical steelsheet having very high magnetic flux density is obtained in a consistentand stable manner.

Examples of the present invention will be set forth under.

EXAMPLE 1 A silicon steel ingot containing 0.038% of carbon, 2.98% ofsilicon, 0.006% of sulfur, 0.03% of selenium, and 0.1% of arsenic wasbroken down into 200 mm thick slabs. The slabs were heated to l250-"-Cand then subjected to continuous hot rolling to obtain hot rolled steelcoils of 2.3 mm thickness. During the cooling step of the above hotrolling, adjustments were made so as to maintain the steel coils in atemperature range of l200 to 850C for 200 seconds. The hot rolled steelcoils were acid pickled and some of them was subjected to preliminaryannealing at 900C for 5 minutes and the remainder were not subjected tothe preliminary annealing. Then all of the coils were reduced to 0.30 mmthickness by a single-step cold rolling and subjected to decarburizationannealing and finishing high temperature annealing.

The magnetic properties along the rolling direction of the products thusobtained are as follows.

1. Product not subjected to preliminary annealing L935 wb/m L18 watt/kgMagnetic flux density:

Watt loss:

2. Product subjected to preliminary annealing 1.965 wb/m 1.10 watt/kMagnetic flux density:

Watt loss:

EXAMPLE 2 Silicon steel ingot containing 0.045% of carbon, 3.01% ofsilicon, 0.004% of sulfur, 0.05% of selenium and 0.05% of arsenic wasbroken down into slabs of 200 mm thickness. The slabs were heated to1300C and subjected to continuous hot rolling to obtain hot rolled steelcoils of 2.3 mm thickness. During the cooling step, the hot rollingadjustments were made so as to maintain the steel coils in a temperaturerange of l200 to 850C for 300 seconds. The thus obtained steel coilswere acid pickled, cold rolled to an intermediate thickness of 1 mm,subjected to intermediate annealing at 850C for 5 minutes in hydrogenstream and then secondary cold rolling to obtain the final thickness of0.30

mm. The cold rolled steel sheet thus obtained was subjected todecarburization annealing and finishing high temperature annealing. Themagnetic properties along the rolling direction of the product are asfollows.

1.92 wb/m l.l2 watt/kg Magnetic flux density: Watt loss:

u w l'llbll carbon, 2.5 to 4.0% of silicon, 0.01 to 0.10% of seleniumand one of an element selected from the group consisting of 0.01 to0.15% of arsenic, 0.02 to 0.3% of bismuth, 0.02 to 0.3% of lead, and0.02 to 0.2% of antimony with a sulfur content not more than 0.01% tocold rolling in which the reduction of the final cold rolling step isnot less than then decarburization and final annealing.

2. The process according to claim 1 in which the silicon steel ingotcontains two or more than two of an element selected from the groupconsisting of arsenic, bismuth, lead and antimony in a total amount of0.02 to 0.5%.

1. PROCESS FOR PRODUCING A HIGH MAGNETIC FLUX DENSITY GRAINORIENTEDELECTRICAL STEEL SHEET COMPRISING: SUBJECTING SILICON STEEL INGOTCONTAINING 0.005 TO 1% OF CARBON, 2.5 TO 4.0% OF SILICON, 0.01 TO 0.10%OF SELENIUM AND ONE OF AN ELEMENT SELECTED FROM THE GROUP CONSISTING OF0.01 TO 0.15% OF ARSENIC, 0.02 TO 0.3% OF BISMUTH, 0.02 TO 0.3% OF LEAD,AND 0.02 TO 0.2% OF ANTIMONY WITH A SULFUR CONTENT NOT MORE THAN 0.01%TO COLD ROLLING IN WHICH THE REDUCTION OF THE FINAL COLD ROLLINGG STEPIS NOT LESS THAN 70%, THEN DECARBURIZATION ANDFINAL ANNEALING.
 2. Theprocess according to claim 1, in which the silicon steel ingot containstwo or more than two of an element selected from the group consisting ofarsenic, bismuth, lead and antimony in a total amount of 0.02 to 0.5%.